Such an NMR probe head is known, for example, from DE 10 2013 201 110 B3 (=reference [1]).
NMR spectroscopy is a powerful process for instrumental analysis which can be used to determine, in particular, the chemical composition of measurement samples. In this case, RF (=radio-frequency) pulses are irradiated into the measurement sample, which is in a strong static magnetic field, and the electromagnetic reaction of the sample is measured.
In order to reduce line broadening on account of anisotropic interactions, it is also known to allow an NMR sample of solids to rotate during the spectroscopic measurement, tilted at the so-called “magic angle” of approximately 54.74° relative to the static magnetic field (“MAS”=Magic Angle Spinning). The MAS rotor is arranged in an MAS stator using gas bearings, is subjected to temperature control and is caused to rotate with a second gas flow. This means that NMR MAS probe heads comprise a stator which is operated using compressed air and comprises a plurality of compressed gas supplies for said purposes, which compressed gas supplies are separate from one another and are typically led from a pedestal box of the probe head to the measurement region, the actual MAS module.
The present invention relates to an NMR probe head (that is to say generally also for NMR applications without MAS) for introduction into the bore of the magnet system of an NMR spectrometer. The central component in the probe head is the resonator which is used to receive the signal from the measurement sample. The signal is typically relatively weak, and accordingly one of the most important aims is to increase the sensitivity. An additional difficulty is that electromagnetic fields which enter the measurement arrangement from the outside additionally influence the measurement quality.
In order to electromagnetically shield the RF network of the NMR arrangement from the environment, an RF seal is frequently used. The probe head therefore has a generally cylindrical shielding tube in which RF electronic components, in particular RF resonator coils, and possibly the MAS turbine are accommodated, wherein the MAS turbine is arranged in the region of that end of the shielding tube which faces away from the pedestal box. The probe head, with its shielding tube, is typically inserted from below into the vertical room temperature bore of a usually superconducting magnet, is positioned therein and is held therein using hooks, supports, screws or the like. The NMR sample or the MAS turbine is then precisely in the magnetic center of the magnet.
In the concept used in U.S. Pat. No. 5,262,727 A (=reference [2]) , the RF seal is placed at the very bottom in a base disk, but complete sealing is not ensured. In addition to the RF tightness, however, good electrical ground contact is also desired. However, in this prior art, it is not certain whether absolute sealing can thus be ensured. Reference [2] discloses, in particular, RF shielding of an RF coil in an NMR probe head comprising cylindrical shields and shielding disks which surround the measurement sample. The additionally required components which are poorly suited to MAS applications are disadvantageous here. There is no rigid backbone, in particular no waveguide array.
U.S. Pat. No. 6,329,820 B1 (=reference [3]) describes an NMR probe head having a shielding tube and a metal-coated separating plate for the purpose of shielding. The shielding tube is permanently connected with metal wires which have been soldered on. A separate backbone and a removable shielding tube are not disclosed.
Patent Abstracts of Japan 05307073 A (=reference [4]) describes RF shielding for an NMR probe head that has a shielding tube which is in the form of a circular waveguide and is appropriately dimensioned for RF attenuation. However, the probe head does not have a separate mechanical backbone.
The German patent application DE 10 2017 208 841.9 (=reference [5]) by a sister company of the applicant was still unpublished on the present filing date. In this concept of the so-called iProbe, the RF seal has been placed at the halfway point for conceptual reasons. This NMR probe head comprises a device for shielding against external RF radiation having an electrically conductive shielding tube which is arranged along the z axis and can be pushed onto a base disk in the z direction around the RF coils and the RF network. A shielding disk is provided at an axial distance from the base disk, and a tensible RF seal is arranged between this shielding disk and the shielding tube in the mounted state and is geometrically configured in such a manner that, in a first mounting state, the shielding tube can be pushed over the shielding disk in an unbraced and force-free manner and, in a second mounting state, the RF seal can be mechanically braced between the shielding disk and the shielding tube in such a manner that an electrically conductive connection to the shielding tube is ensured over the periphery of the shielding disk. However, the disadvantage here is the complicated production of the RF seal.
Furthermore, however, there is also a need for yet additional structural components which provide the probe head with the necessary stability, with the result that the measurement sample (the MAS rotor in the case of MAS applications) is always in the optimum region (and also at the exact magic angle for the MAS) of the magnet. Rigid metal rods are conventionally used as connections between the stator and the pedestal box and are provided, at regular intervals, with metal carrier plates (so-called “frames”). This structure provides the required mechanical stability. However, the disadvantage here is that both the carrier plates and all connecting rods have to be connected to the surrounding shielding tube in an electrically conductive manner at short distances. Without this grounding, natural resonances of the frame elements would be excited by the internal probe head circuit (“internal interference”) and external interference signals would be injected into the sample chamber (“external interference”). The latter is caused by the fact that the presence of non-grounded metal components inside the shielding tube reduces the so-called cut-off frequency of the latter, with the result that all interference at the open end of the tube is passed into the interior of the latter.
US 2014/0167756 A1 (=reference [6]) discloses an NMR MAS probe head in which the typical complicated structure of the numerous connecting lines, trimmers, etc. between the pedestal box and the MAS module and the spatial vicinity of mechanical and electronic components can be seen. This embodiment is the “conventional” structure of an NMR MAS probe head.
DE 10 2005 025 012 B4 (=reference [7]) describes a modular MRI probe head, wherein a base part and channel modules together have an elongated structure, wherein the channel modules contain bushings through which RF components but also supply gases can be conducted. The probe head presented here is not produced from a conductive material and is also not used to separate mechanical and electronic components since it is not designed for use in NMR MAS technology.
Reference [1] cited at the outset discloses an NMR MAS probe head having a transport line for the measurement sample which is brought into the MAS stator from below. Said document discloses, in paragraph [0052], that a probe head frame is formed in the interior of the tube, on which frame the stator and various electronic components (not separately illustrated) for an NMR measurement on a measurement sample arranged in the stator are arranged. The first section of the transport line is in the form of a rigid tube here, as a result of which the frame achieves improved stability in comparison with the prior art known until then.
In contrast, however, spatial separation of the mechanical supporting structure and the supply lines from the electronic components, in particular the RF components in the probe head, with simultaneous simplification of the design and an increase in stability would also be desirable.